Identification of co-expressed gene sets for major molecular initiating events in rodent liver carcinogenesis using wild type and knock out rats
Liver tumors are the most common carcinogenic outcome observed in two-year bioassays used for regulatory risk assessment of chemicals and pharmaceuticals. Because these traditional long-term studies are resource-intensive and not feasible for most chemicals, there is a critical need to develop alternative approaches that can identify potential rodent liver carcinogens in short-term studies (e.g., [≤] 1 month). Biomarkers associated with molecular initiating events (MIEs) of early rodent liver tumorigenesis may enable the prediction of two-year outcomes from such studies. Here, we identified transcriptional changes associated with six key initiators of rat liver tumorigenesis: five transcription factors (TFs) associated with non-genotoxic modes of action (AhR, PXR, CAR, PPAR, and ER), and tumor suppressor protein p53, which is activated by diverse cellular stressors including genotoxicity. Three known chemical activators for each TF were administered orally once daily for five days at doses associated with tumorigenic responses in previous rat cancer bioassays. RNA-seq identified differentially expressed genes (DEGs) that were consistently regulated by at least two activators in wild-type but not in corresponding TF knockout animals. Gene set enrichment analysis of ranked differential expression results demonstrated enrichment of pathways reflecting established TF biology. When comparing the consensus DEGs for each TF to a library of chemically induced, liver-specific gene expression profiles, we found that in most cases the chemicals with the most similar profiles to the consensus lists were known to activate the corresponding TF. Collectively, these findings identified candidate gene sets that may have utility for monitoring the induction of carcinogenesis-associated MIEs in short-term studies.